1. Technical Field
The present invention relates to an image sensing apparatus capable of reading out charge from a photoelectric conversion element using both destructive and non-destructive readout modes.
2. Related Art
Recently, a monitoring camera mounted on a mobile object is attracting attention. For example, there is known a monitoring camera, known as an on-vehicle camera, which is mounted on a vehicle. The on-vehicle camera may be used in detecting a centerline or guardrail on a road or rail, an inter-vehicle distance to a preceding or following vehicle, and an abrupt change in contrast at the entrance to a tunnel. The recognition or detection result is output to a control unit of a predetermined control object (for example, brakes, steering, or alarm systems), and the control unit controls the control object on the basis of the detection result and takes measures against unforeseen changes in situation or accidents that may be caused by a person's carelessness.
The on-vehicle camera may also be used in detecting the entire states of a photographic object that cannot be caught with human eyes' capability and thus enabling a high-speed control of the control object based on the detection result.
The high-speed control can find its application in a vehicle moving at a high speed, for example, in high-speed detection of turning on and off of a preceding vehicle's brake lamp and thus (automatically) braking of the vehicle or in (automatically) avoiding dangerous objects flying or rushing into the vehicle. In a mobile object such as the vehicle moving at a high speed, it is desirable to immediately catch (detect) instantaneous changes in state so as to automatically actuate a corresponding mechanism to allow for braking of the vehicle or avoid any possible accident. It is also desirable to stably keep track of other vehicles moving in its forward or side areas and thus enable to predict the movement of a target object and inform the vehicle's driver using a warning signal.
In the related art, there is known a camera for use in factory automation (FA). In such a camera, there is used a technique in which an interframe difference image (background) is taken and a change in the difference image is detected. However, since the background image sensed by the on-vehicle camera cannot be specified and is likely to change with time, it is difficult to use the technique in the high-speed control.
By the way, a recently developed low-cost and high-speed camera has made the high-speed and stable control possible and thus applied in the field of robot vision. The camera can sense images at a frame rate of 1/1000, for example. In a high-frame rate video, there is little movement of the photographic object in one frame, a frame has a high correlation with a preceding or subsequent frame, and the target object (an areal image) in the photographic object shows a small amount of movement. Therefore, the level of pixels corresponding to the frame difference image becomes small. Moreover, it is possible to detect edges on the target object's areal image of the frame difference image caused by the movement (since the width of the edge portion in the difference image becomes small). In addition, it is possible to detect the movement (relative movement) of the target object with improved precision. Accordingly, it allows a device having the camera mounted thereon to immediately detect the target object and the change (movement) in the target object with good precision. That is, with the difference processing of the high frame rate video, it is possible to detect edges with easy and high precision and thus easily estimate the shape of the target object. In addition, it is possible to estimate the change in the shape of the target object in a reduced cycle. Since this application is directed to control of a camera, the coordinate (position) information and shape on the target object serve as an important factor, but matters such a gradation level or a wide-angle image are not crucial to the application (because it only needs to detect an areal image of the target object at a high speed, accurately detect an edge portion, and recognize the shape of the target object).
As an object of the on-vehicle camera, as described above, the camera aims to detect the centerline, the inter-vehicle distance to a preceding vehicle and the like. For this detection, an image needs to have a high contrast and a greater degree of signal-to-noise (S/N) ratio. In addition, video images recorded by a drive recorder before and after an accident need to have sufficient visibility and thus require a sufficient exposure time. That is, it is anticipated to acquire the entire images with a wide angle and a good gradation level (in order to acquire a clear image).
In the end, the on-vehicle camera requires a high visibility (S/N) as well as a high speed for use in a machine vision.
As a known technology for recognizing an object moving in a forward or side areas of a vehicle on the basis of images sensed by the on-vehicle camera, there is known an object recognition apparatus as disclosed in JP-A-7-078258, for example.
The known technology disclosed in JP-A-7-078258 can recognize a predetermined kind of object from objects displayed on a screen of the images sensed by the on-vehicle camera. More specifically, an area on which an object for recognition is supposed to be present is cut off from the screen, and images in the cut-off area are subjected to a recognition process, thereby improving processing efficiency.
However, in the known technology disclosed in JP-A-7-078258, it can be seen from the detailed description of embodiments that the sensed image for use in the recognition needs to have some degree of visibility. That is, since the photographic object is sensed at a frame rate (exposure time) providing a sufficient exposure time, it may be difficult for the on-vehicle camera to acquire a high frame-rate image for use in a high-speed control. Thus, in order to implement the high-speed control in the known technology, there is a need to use two cameras, one for the high-speed control and the other for the visibility. Since the use of two cameras may cause a problem of increasing costs or power consumption, it is desirable to implement with a single camera. However, when an image is acquired at a high speed in the single camera implementation for a stable control, the acquired image may have a poor S/N characteristic due to an insufficient exposure time and it is thus impossible to acquire an image having a quality for use in other purposes.